1. Field of the Invention
The present invention relates to a driving method for a cholesteric liquid crystal display, more particularly, to a single polarity driving method and a non-symmetric driving method for a cholesteric liquid crystal display.
2. Description of the Related Art
Referring to FIG. 1, a reflective cholesteric liquid crystal display 1 mainly comprises: a transparent glass 11, a plurality of liquid crystal units 12 and a light-absorbing glass 13. When a voltage is applied to the display 1, liquid crystal units 12 of the reflective cholesteric liquid crystal display 1 will arrange according to the applied voltage to show image (as shown in the middle diagram of FIG. 1). When there is no applied voltage, the reflective cholesteric liquid crystal display 1 has two stable states: a planar texture and a focal conic texture.
The planar texture is a bright state, that is, the liquid crystal units arrange with a rule on the turn (as shown in the left bottom diagram of FIG. 1), and the outside light can be through the transparent glass 11, the liquid crystal units 12 and the light-absorbing glass 13 with half quantities reflect. Therefore, the reflective cholesteric liquid crystal display 1 is usually utilized in electronic-Book etc., which does not need to often switch over the screen and can show the image using the outside light without the need of the applied voltage so as to save energy.
The focal conic texture is a dark state. In the dark state, the liquid crystal units 12 irregularly arrange (as shown in the right bottom diagram of FIG. 1), and the outside light disorderly enter and are completely absorbed by the light-absorbing glass 13. When there is no applied voltage, the stable state of the reflective cholesteric liquid crystal display 1 is determined by the previous applied voltage.
Referring to FIG. 2, the reflective cholesteric liquid crystal display comprises a plurality of pixels P11, P12, P21 and P22 to show image. The pixels are controlled by a plurality of column electrode C1, C2 and a plurality of row electrodes R1, R2. The pixels are disposed on crossing areas between the column electrodes and the row electrodes. For example, the pixel P11 is controlled by an applied signal combined from the column electrode C1 and the row electrode R1.
Referring to FIG. 3, in the prior art, the applied signal of the row electrode and the column electrode is usually a square wave. The applied signal of the pixel P11 equals the row signal of the row electrode R1 minus the column signal of the column electrode C1, and the applied signal of the pixel P21 equals the row signal of the row electrode R2 minus the column signal of the column electrode C2. In the period t1, the applied signals of the pixels P11 and P21 are initial signals being square waves having positive amplitude and negative amplitude.
By utilizing the square wave having positive and negative amplitude, the conventional AC driving method can avoid the bad degraded affect to the liquid crystal driven by the direct voltage. However, the AC driving method has no help to the switching speed of the pixel. For example, the drivers applied to the column electrode and the row electrode can bear a withstand voltage of 40V, that is, the drivers applied to the column electrode and the column electrode can supply a maximum voltage of 40V. Then, the applied voltage of the pixel is ±40V. However, considering root mean square value, the root mean square value of the pixels is still 40V. Therefore, the root mean square value of the maximum applied voltage of the pixels is the same as the withstand voltage applied to the column electrode and the row electrode. Besides, the switching speed of the pixel is proportioned to the root mean square value of the applied voltage of the pixel. Accordingly, the conventional AC driving method cannot improve the switching speed of the pixel.
Therefore, it is necessary to provide a driving method so as to solve the above problem.